Meandering Turbidite Channels and the Effective Depth of Turbidity Currents

Mason Dykstra¹ and Ben Kneller^2
1 University of California, Santa Barbara, CA
² University of Aberdeen, Aberdeen, United Kingdom

Some deep marine channels show striking similarities to fluvial channels, despite the major differences in the properties of the flows that they conduct. We report some field obsevations from channel deposits within a late Cretaceous paleocanyon in the Rosario Formation of Baja California, Mexico, that suggest that these similarities may be more than superficial. These channel deposits contain packages of inclined heterolithic stratification, formed from sandstone and conglomerate, in mutually erosive sets of a few meters thickness, that are similar in many ways to fluvial point bar deposits. The inclined sets define laterally migrating and sinuous channels locally at a high angle to the confining canyon. Paleocurrent data, taken mostly from clast imbrication in conglomerates, indicate current modes along the channel thalweg, but with secondary flow either up or down the point bar. We suggest that at times the lower part of the turbidity currents flowing down the channels is behaving similarly to within-bank fluvial flows, with a cross-channel component of flow towards the cut bank, and return flow at the bed sweeping up the point bar. At other times this secondary circulation is reversed. We also suggest that the lower, within-channel part of the flow is decoupled from the superjacent flow, and has an effective depth that is a function of its internal Froude number.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005